The present invention relates to a method for making polycarbonates exhibiting liquid crystalline properties. More particularly, the present invention relates to liquid crysytalline polycarbonates which are prepared by effecting in the melt, the transesterification of diphenylcarbonate with a mixture of methylhydroquinone and 4,4'-dihydroxybiphenyl.
Prior to the present invention, a number of liquid crystalline polyester-carbonates were reported, as shown by U.S. Pat. No. 4,107,143 and U.S. Pat. No. 4,371,660. Roviello et al., European Polymer Journal 15 (1979) pp. 423-430, reported the first liquid crystalline polycarbonates in 1978. These polycarbonates were prepared from 4,4'-dihydoxy-.alpha.,.alpha.'-dimethyl-benzalazine and various .alpha.,.omega.-alkanediol bischloroformates. Although these materials were found to be anisotropic under melt conditions, their melting points were less than 220.degree. C. Additional liquid crystalline polycarbonates are shown by Mahabadi et al., U.S. Pat. No. 4,543,313; Sato et al., Makromol. Chem., Rapid Commun. 7 (1986) pp. 231-234. Japanese patent application No. 5.61-264020, uses a mixture of 70 mole % of 4,4'-dihydroxybiphenyl, and 30 mole % of bisphenol-A, and employs transesterification with diphenylcarbonate under melt conditions.
The present invention is based on the discovery that polycarbonate exhibiting liquid crystalline properties at temperatures exceeding 300.degree. C., can be prepared in the melt by a transesterification reaction between diphenylcarbonate and mixtures of methylhydroquinone and 4,4'-dihydroxybiphenyl. The polycarbonates are found to be insoluble in common solvents, such as chloroform, methylene chloride, toluene, orthodichlorobenzene and 1,1,2,2-tetrachloroethane.